If i am reading correctly, you need to consider doing the following.
Contact the lending institution and get a 100% loan offer immediately.
Instruct your solicitor to return the contracts to vendor solicitor.
If the 20k is a loan from your sister, repay her from the surplus on completion of the sale.
If it is a wedding present, i cannot understand why the solicitor is concerned.
It appears that he want to charge stamp duty on the purchase price. You could contact the revenue and check the situation.